Speaking outside Downing Street hours after the announcement, the British politician said he had been informed of the engagement via a piece of paper that was passed to him during a cabinet meeting.

He added that he had since spoken to the groom-to-be, whom he said was "extremely excited" about his forthcoming marriage.

Mr Cameron is by no means the only public figure sending his congratulations to William and Kate.

The Queen, William's grandmother, has said she is "absolutely delighted", while his father Prince Charles said he is "thrilled, obviously" about the engagement. "They've been practising long enough," he added.

While on her way to a London theatre, William's stepmother, Camilla the Duchess of Cornwall, told reporters: "It's absolutely brilliant, isn’t it? It's absolutely wonderful."

Sophie, Countess of Wessex has described the news as "absolutely thrilling", and Earl Spencer, William's uncle and the brother of Diana, said: "It's wonderful news. Very exciting. My family are all thrilled for them both."

Messages of support have also been coming in from other quarters.

Davina McCall has been on her Twitter page to share her thoughts, writing: "Yay! William and Kate engaged, A royal wedding! Hoorah! Hope they've got my address…"

Radio DJ Fearne Cotton has written: "Congratulations to William and Kate, and Mayor of London Boris Johnson added: "Really pleased for Prince William and Kate."

The news has reached across the Atlantic too. Welsh singer Katherine Jenkins has already tweeted: "Waking up in NYC to the wonderful news.,. Congratulations to Prince William and Kate Middleton on their engagement."

And Melissa Joan Hart – former star of Sabrina the Teenage Witch – said: "Yay Kate! New princess for us to adore! Can't wait to see the dress!"